The transfer of energy to and from a working fluid typically is accomplished under the control of a condition sensing device such as a temperature responsive unit or a pressure responsive unit. Ordinarily, the condition responsive means measures a single condition of the working fluid and in turn controls the rate of transfer of energy to or from the working fluid in proportion to the deviation from a set point. This type of control system typically has a proportional offset which is an offset from the desired setpoint or control point established for the operation of the system.
In many systems, there is a minimum or fixed lowest possible energy transfer rate for the system. Above that minimum rate, the system typically can modulate continuously to some fixed upper limit. There are startup energy losses associated with the transition between a complete off state and the lowest operating rate, and therefore each time the system is caused to cycle there is a significant startup loss.
The startup losses, and the operation of the system with a proportional offset, typically leads to certain inefficiencies. A more efficient manner of operating such a system can be brought about by minimizing the number of startup times for the system, and by tailoring the operation of the control so that the working fluid is not over heated to supply just the minimum amount of energy required to satisfy a particular load.
A description of a prior art type of condition control system will be described in the section of the application entitled "Description of the Preferred Embodiment" with reference to FIG. 1 which is identified as prior art. This description will establish what some of the prior art is, and will show why that type of prior art control system is deficient as relates to an efficient manner of operating a condition control system. The system that will be described will specifically be a boiler supplying steam to a steam heated load in response to a fuel burner control system. A further prior art system which provides an economical and efficient manner of operating is also disclosed in U.S. Pat. No. 4,373,663 issued Feb. 15, 1983, to J. M. Hammer.
The present description will be directed primarily to boilers in which water is converted to steam and then applied as the working fluid to a load. Under these conditions, a pressure sensor determines the condition of the working fluid and this type of system operates with a fuel burner that is initially operated to a low fire rate, and then released to a high fire rate. Typically this type of system operates in a modulating manner between the two fixed rates in order to satisfy the demand for steam from the boiler. The pressure sensor regulates the burner. This type of system is inefficient in that each time the burner starts, losses accompany the startup, and further the system is inefficient in that the pressure sensor normally provides a much higher pressure than is necessary to efficiently satisfy the load.